|Mary Peterson and Elizabeth Salvagio (2010), Scholarpedia, 5(4):4320.||doi:10.4249/scholarpedia.4320||revision #91260 [link to/cite this article]|
For two contiguous regions in the visual field, the common perceptual outcome is that the edge between them appears to be a boundary for only one of them, and that region—the figure—appears to have a definite shape. The contiguous region—the ground—appears shapeless near the edge it shares with the figure, and is perceived to continue behind it. Thus, in addition to being shaped, the figure appears nearer than the ground part, involving depth perception, and the ground appears to be occluded by the figure. This perceptual experience is labeled figure-ground perception. An example is shown in Figure 1, where the edge shared by the black and white regions appears to enclose the black region and the white region appears to continue behind the shaped black region. Figures constitute the objects we perceive and with which we interact. Therefore, figure assignment—the determination of which portions of the input correspond to figures—is a critical component of perception. (The ground is often shaped by edges at some distance from the figure. For instance, even though the white region in Figure 1 is unshaped near the border it shares with the smaller black region, it is shaped by the outline border it shares with the larger surrounding white region. As this example makes clear, a region can be a ground along some portion of its bounding edges, and a figure along other portions.)
Figure assignment is not simply given in the input; it results from processes of perceptual organization. Many factors influence figure assignment; some of these factors have been known since the early 20th century, whereas others have been identified in the early 21st century. Figure-ground perception has been studied most extensively in vision, although there is some research on tactile (Kennedy 1993) and auditory (Bregman 1990) figure-ground perception.
Factors that influence figure assignment
Edgar Rubin and the Gestalt psychologists, who first brought figure-ground perception to the attention of perception psychologists early in the 20th century identified some of the visual properties associated with figures rather than grounds; these properties are now known as “classical configural cues.” The term “configural” applies because these cues predict which of two contiguous regions in the visual field will appear to be configured, or shaped (versus unshaped).
Classical configural cues
Regions that are convex, symmetric, smaller in area, enclosed, or surrounded are more likely to be seen as figure than contiguous regions that are concave, asymmetric, larger in area, or surrounding. These configural properties (or “configural cues”) are illustrated in Figures 2a-2d. In Figure 2a, the black regions have convex parts, whereas the white regions have concave parts. In Figure 2b, the black regions are smaller in area than the white regions. In Figure 2c, the black regions are symmetric around a vertical axis, whereas the white regions are asymmetric. And, in Figure 2d, the black region is enclosed and surrounded by the white region. In displays like these, observers are more likely to perceive the black regions as the shaped figures, and to perceive the white regions as backgrounds. Note that if the white, rather than the black, regions were convex, symmetric, smaller, or enclosed they would be seen as figures; these effects do not depend upon the contrast polarity of the regions relative to the overall background. Contrast with a background is, however, a cue for depth perception in that lower-contrast objects appear more distant than higher-contrast objects (O’Shea et al. 1994). Thus, when testing the effectiveness of potential configural cues, it is important for contiguous regions to have equal contrast with the overall background. We have achieved this by using black and white regions on a medium gray overall background in Figure 2a - Figure 2d.
The importance of these classic configural properties for figure-ground perception was originally revealed via demonstrations (e.g., Koffka 1935; Kohler 1929/1947; Rubin 1915/1958). Empirical studies have tended to support those demonstrations (e.g., Kanisza & Gerbino 1976), although some caveats apply. For instance, experiments assessing the effectiveness of symmetry as a configural cue have produced equivocal results (e.g., Pomerantz & Kubovy 1986). In addition, the effectiveness of convexity as a configural cue has recently been shown to vary with context (Peterson & Salvagio 2008).
The classical configural cues were all properties that could be measured on the image; they were geometric in that they were properties of the simple rectilinear or curvilinear lines or shapes in the displays. The Gestalt psychologists held that these cues were largely innate, and did not depend fundamentally upon an individual’s past experience (Wertheimer, 1923). In support of this claim,
- They showed that novel regions possessing the configural properties were seen as figures (see Figure 1 and Figure 2d, for instance). Evidence that figure-ground perception can proceed without input from past experience for novel shapes does not eliminate the possibility that past experience also exerts an influence when shapes are familiar, however.
- They demonstrated widespread use of the classical configural cues by adults, at least when displays were exposed for long durations. Such findings cannot demonstrate innateness, however, because high agreement between adults can arise because of learning.
Thus, it is unclear whether responses to these configural properties per se are innate, or whether a sophisticated learning mechanism has evolved that allows humans to extract the statistical properties of the environment in which they live (and configural cues are among those properties). See also Grossberg & Swaminathan (2004) for a model that uses statistical learning to account for some perceptual effects.
Non-classical geometric configural properties
Recent experiments tested directly whether or not past experience could affect figure assignment, and found that it could, contrary to the Gestalt claim (e.g., Peterson et al. 1991; Peterson & Gibson 1994). In the displays used in these experiments a portion of a familiar, nameable, object was suggested on only one side of an edge; hence, the shapes potentially perceived on opposite sides of the edge differed in familiarity. Observers are more likely to perceive the figure on the side of the edge where the familiar object lies when the display is presented with the familiar object in its typical upright orientation (see Figure 3a) rather than in an inverted orientation (see Figure 3b). (For review see Peterson 1994). Orientation dependency was critical for attributing these effects to past experience rather than simply to geometric properties because the former but not the latter would vary with a change from upright to inverted.
Experiments also showed that effects of familiarity were observed only when the parts of the familiar object were shown in their proper spatial relationships (e.g., when the parts representing the standing woman were arranged from top to bottom as they are encountered in the world: a head, shoulders, torso, skirt, legs). Effects of familiarity were not observed when the parts were rearranged (e.g., see Figure 3c where the part representing the skirt is on top, the part representing the head is on the bottom, and the torso and legs are in between). These effects necessarily depend upon past experience and as such may seem to differ from effects attributable to the classical configural cues because past experience is not always instantiated as geometric relationships, whereas the configural cues are. However, we note that in the experiments we have described, past experience is operationalized as a familiar configuration of parts which can be specified geometrically. It may not be the case that all forms of past experience influence figure assignment but only those that are embodied geometrically.
Peterson and Gibson (1994) showed that familiar configuration can affect figure assignment even when it conflicts with classic Gestalt configural cues. Consider displays like Figure 3d, where the asymmetric black region portrays a familiar object (a seahorse) whereas the symmetric white region portrays a novel shape. Here, the cues of familiar configuration and symmetry compete with each other. When an upright version of this display is exposed briefly, familiar configuration is slightly more powerful than symmetry, but the two cues seem to compete so that the shaped figure is sometimes seen on the unfamiliar symmetric (white) side of the central edge. These results showed that familiar configuration does not invariably dominate other cues. Instead, it is one of many visual properties used for figure assignment (Peterson 1994).
- Regions with a wide base are more likely than regions with a narrow base to be seen as figures (see Figure 4a; Hulleman & Humphreys 2004).
- The lower of two regions separated by a horizontal border is more likely than the upper region to be seen as the figure (see Figure 4b; Vecera et al. 2002).
- A region that protrudes into a contiguous region is likely to be seen as the figure (see Figure 4c; Hoffman & Singh 1997).
The configural cues are shape cues; they determine where the shape lies with respect to an edge. But recall that the region complementary to the figure is often perceived to complete behind it. The perceptual completion of the ground has not received much attention in the study of figure-ground perception. It is possible that at least some of the configural properties may convey depth information as well as shape information (Burge et al. 2005; Grossberg, 1994; Kanizsa, 1985; Nakayama, Shimojo, & Silverman, 1989), and that perceptual completion may be most compelling when those cues are present (see, for instance, Peterson & Salvagio 2008).
The region that appears shaped also tends to appear closer (although this relationship does not always hold, e.g., Palmer 1999; Peterson 2003). Depth cues determine which of two contiguous regions is closer to the viewer even in the absence of the classic configural cues. Closer regions tend to be shaped by the edges they share with contiguous regions in the visual input, and the latter typically appear to continue behind as backgrounds. There are ample empirical investigations of the depth cues: for instance, research investigates the ranges over which different depth cues are most effective (e.g., Cutting & Vishton 1995) and the rules by which depth cues combine (e.g., Landy et al. 1995). Very little research investigates how configural cues and depth cues interact (but see Bertamini, Martinovic, & Wuerger, 2008; Burge et al. 2005; Dresp et al. 2000; Peterson & Gibson 1993). Such research is needed for a full understanding of figure-ground perception.
Subjective factors can also influence figure assignment. For instance, the viewer’s intention to perceive one of two contiguous regions as figure affects figure-ground perception (e.g., Peterson et al. 1991). And regions at which the viewer is looking (fixated regions) are more likely to be seen as figures than adjacent un-fixated regions (Peterson & Gibson 1994). Similarly, an attended region is more likely to be seen as figure than the complementary unattended region, even without fixation (Baylis & Driver 1995; Vecera et al. 2002). Subjective factors can alter the likelihood of seeing the figure on one side of an edge, but typically they tend not to overpower configural cues.
A region filled with a high spatial frequency pattern is more likely to be seen as the shaped figure than a contiguous region filled with a low spatial frequency pattern (see Figure 5a; Klymenko & Weisstein 1986).
An extremal edge (EE) is a self-occluding edge. When shading and texture gradients are used to depict an extremal edge along one side of a border but not the other, observers show a strong bias to report seeing the EE side as nearer than the non-EE side (Palmer & Ghose 2008). A sample is shown in Figure 5b, where the extremal edge lies on the left side of the central border. Research is needed to determine whether extremal edges are depth cues, figural cues, or both.
Consider a region bounded by two thin colored lines that are parallel to and touching each other. One of the colored lines contrasts less with the background than the other. Pinna, Werner & Spillmann (2003) showed that under these conditions the low contrast color spreads orthogonally from the line and fills the bounded region; they called this phenomenon the “Watercolor Illusion.” They showed that the region through which color spreads is more likely to be seen as the figure than it would be without the color. Not much is known about the Watercolor Illusion as a figural cue; unlike other figural cues, it has not been examined in isolation, it has always interacted with one or more of the other figural cues.
Ambiguous figure-ground perception
Figure-ground perception can be ambiguous. The best-known example of an ambiguous figure-ground display is Rubin’s vase-faces stimulus; an adaptation of the original image is shown in Figure 6. In this display, viewers can perceive either the central white region or the surrounding black region as the figure at any moment. When the white region appears to be the figure, it has a definite shape, one that resembles a white vase or a goblet. The factors that favor seeing the white region as figure include partial symmetry, small area, closure, and enclosure. When the outer black regions appear to be the figures, they have definite shapes, ones that resemble two profiles of people facing each other. The factor of familiarity favors perceiving the black regions as figures. (Global symmetry of the black regions my also play a role.)
Observe how the black regions in Figure 6 appear shapeless when they are seen as grounds to the white vase, yet they appear shaped like profiles of faces when they are seen as figures. Similarly, observe how the white region appears shapeless when it is seen as the ground to the black profile faces, yet appears to be shaped like a vase when it is seen as figure. Thus, regions appear shapeless (at least near the edge they share with figures) when they are seen as grounds even though the same regions appear shaped when they are perceived to be figures.
How does figure-ground perception occur?
The apparent shapelessness of the regions adjacent to figures has led to the proposal that figure-ground perception results from a winner-take-all competition. Recent behavioral evidence shows that competition does occur (e.g., Peterson & Lampignano 2003; Peterson & Enns 2005). But what is competing? Some models propose that the competition occurs between edge units facing in opposite directions that exist everywhere in the visual field (Kienker et al. 1986; O’Reilly & Vecera 1998; Vecera et al. 2000). The edge units that win the competition are perceived as the boundaries of the figures, whereas those that lose the competition are suppressed. Other theorists contend that candidate shapes – that is, shapes that might be seen on opposite sides of an edge -- compete directly with each other. The winning shape is perceived as the figure, whereas the losing shape is suppressed (Peterson & Gibson 1994). Consistent with the shape competition hypothesis, recent evidence indicates that shapes that are suggested, but not perceived, on the ground side of an edge are suppressed (Peterson & Skow 2008). Many figure-ground percepts can be modeled in terms of interactions between 3D boundary and surface representations that obey computationally complementary rules (Grossberg, 1994, 1997; Kelly & Grossberg, 2000) that include both cooperative and competitive interactions.
Experimental work investigating the competition is relatively new. More studies directed to uncovering the nature of the competition are necessary. For instance, experiments investigating how depth cues alter the between-shape competition are needed to elucidate the mechanisms of figure-ground perception.
1. When two regions share a border, perceptions other than figure-ground perception can and do occur (Kennedy 1974). For instance, the shared border can appear to be an edge of a three-dimensional object, and the two regions could be perceived as different faces or surfaces of that object. Alternatively, both regions might be seen as figures, as in the case of Escher’s prints and repeating tile patterns or striped patterns. Or, one might perceive a shaped hole in that the surface in which the hole has been cut appears to be the near surface, but the hole appears to have shape. Consider, for instance, a hand-shaped hole cut into a piece of metal. How will theories about perception change when we consider these other possible outcomes?
2. Edges separate surfaces in touch as well as in vision, and figure-ground perception occurs (Kennedy 1993). There are analogous percepts in hearing, taste, and smell as well. Do the same mechanisms produce figure-ground perception across the senses?
3. There are many other types of ambiguous figures, including binocular rivalry stimuli and reversible stimuli like the duck/rabbit and the Necker cube. Interactions between cooperative and competitive mechanisms have been proposed to play a key role in explaining many of these reversals. Can comparisons of the different types of ambiguous stimuli shed light on where and how competitive mechanisms operate in the brain?
4. A lot is known about the development of depth perception, but there is little empirical work on the development of the configural cues, perhaps because the Gestalt Psychologists stipulated that they were innate. What is the developmental trajectory of figure-ground perception?
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